The invention is in the field of optical inspection by image processing, and relates to a process and an arrangement according to the introductory parts of the corresponding independent claims for optical inspection of translucent articles, in particular containers, by means of image processing.
The inspection of translucent containers by image processing is known. Such inspection may for example be used for production checks, for reading marks, or for testing returnable containers for cleanliness and absence of defects. For these purposes an image (in the widest sense of the word) is usually produced by a transmitted-light process, that is to say with an arrangement in which at least that part of the container which is to be inspected is positioned between a direct or indirect light source and an image recording apparatus, for example a camera. This image is analysed by known methods involving for example the detection and/or interpretation of dark or black spots in the image. According to the result of the image analysis, control systems may then be activated to bar the container concerned from further processing.
Soiling, marks, defects etc. are usually recognized in the images obtained as dark or black spots on an otherwise generally light background representing a clean, intact, unmarked container wall. However, it has been found that the images may contain dark and black spots due not only to marks, soiling etc. but also to the container wall itself, that is to say due to the geometry and/or surface condition of the wall or to an optical combination of undetectable liquid residues with the container wall geometry, so that, owing especially to reflection at the surfaces of the container wall, the transmitted illumination becomes non-homogeneous, that is to say, the image contains for example dark or black spots which may also shift or fluctuate, especially with arrangements operating continuously at high speed. At these spots, detection of flaws, soiling, marks etc. by means of image processing becomes impossible, or at least very difficult, as the contrast between the background and the image of an object to be detected is reduced or completely lost.